This research proposal addresses questions regarding the perception of complex harmonic sounds in birds. Estrildid finches, in particular the zebra finch (Taeniopygia guttata), have served as models for psychoacoustic, neuroanatomical, and neurophysiological studies of vocal production and perception. Much is known concerning the details of vocal learning, the motor control of song production, and the perceptual capabilities of this species in discriminating natural vocalizations. However, the specific acoustic characters involved in making such discriminations and their relationship to the acoustic properties present in natural calls, have received little or no study. In addition, the perceptual capabilities of related species of estrildids are not known. An additional estrildid species, the red-billed firefinch (Lagonosticta senegala), will be used as a comparison in these experiments. I will evaluate the hearing abilities of these two species of estrildid finches in relation to tones and complex harmonic sounds. Harmonic stimuli provide an important class of sounds that resemble closely the harmonic structure of natural zebra finch songs and calls, but not the pure-tonal songs and calls of several other estrildid species, including the red-billed firefinch. In order to evaluate how pitch cues may be processed, I will measure sensitivity to the relative phase structure of natural vocalizations, relative phase relationships of harmonic complexes, and perceptual categories formed by different types of harmonic mistunings using psychoacoustic programming and testing procedures.